1. Field of the Invention
This invention relates to automatic gain control circuits (AGC) and in particular AGC control of a single gate gallium arsenide (GaAs) FET amplifier.
2. Background Description
In digital radio systems, it is important to use modulation techniques which increase the number of bits per second per Hertz. As a result a number of multi-level modulation techniques have been devised for such use. One effect of these modulation techniques is to require a fairly high degree of linearity in the receiver input sections of such radio systems.
Because of spectrum utilization requirements, the frequencies most often available for digital radio systems are in the 11 GHz range and higher although some systems operate in the 6-8 GHz range. Such frequencies are adversely affected by rain. For example, it is well known that at 11 GHz rain attenuation is a major obstacle to the attainment of long path lengths between repeaters. A detailed study of this phenomena was made and was reported by S. H. Lin in an article, "Statistical Behavior of a Fading Signal", Bell System Technical Journal, Vol. 50, No. 10 December 1971, p. 3211. Because of the rain attenuation margin required as well as other factors, a dynamic operating range in the order of 60 dB appears to be realistic, particularly for the high rain areas. The dynamic range is defined as the difference between the maximum and minimum received signal levels (RSL) for a bit error rate (BER) of 1.times.10.sup.-6. The minimum (RSL) is determined by the noise figure (NF) of the receiver and the signal-to-noise ratio required for the modulation technique employed. The maximum (RSL) depends on the sensitivity of the modulated signal to non-linearity plus resultant AM to PM conversion which is caused by a high RSL. In order to use the available RF spectrum efficiently, higher order modulation schemes must be used. Signals containing amplitude modulation (16 QAM) are obviously more sensitive to amplitude compression than constant envelope signals such as are obtained in a 8 phase modulation technique (8 .phi. PSK). The following table shows the effects that the different dynamic ranges have on the maximum path length at 11 GHz:
______________________________________ Dynamic Operating Tampa, Fla. Wilmington, N.C. Portland, Ore. Range (km) (km) (km) ______________________________________ 60 dB 10.4 11.3 25 55 dB 8.9 9.6 21.3 50 dB 7.5 7.9 18.1 45 dB 6.4 6.9 15 ______________________________________
The sites selected represent the full range of expected conditions: (a) extreme rain rates (b) typical Eastern and Midwestern locations (c) few intense thunderstorms.
From the table above it can be seen that a dynamic range of 60 dB or more is highly desirable, because it determines the maximum useable hop length. Unfortunately this leads to a extremely high RSL for the system. For example, a 16 QAM system with a guaranteed threshold level of -70 dbm will have a maximum receive signal level of at least -10 dbm, a level at which the receiver input must still be linear.
A typical receiver input consists of an RF receive filter, low loss mixer and IF preamplifier with automatic gain control. Although such a receiver is not shown in detail, the elements 1, 2, 4, 6, 14 and 16 as shown in FIG. 1 would make up such a receiver input circuit. A receiver NF between 7 dB and 8 dB can be obtained with such a circuit if the IF preampliflier NF is kept below 1.5 dB. An IF preamplifier with voltage feedback, using a NEC NE64535 transistor and AGC after the input stage gives a typical noise figure of 1.2 dB. The overload characteristic of such a typical receiver input is shown in FIG. 3. The level of inter modulation products (2A-B) from two equal level signals f.sub.A and f.sub.B is used as a measure of linearity of the receiver input configuration. As a result of non-linearity, 2f.sub.A -f.sub.B, 2f.sub.B -f.sub.A, 3f.sub.A -2f.sub.B, 3f.sub.B -2f.sub.A, etc., intermodulation product signals appear at the output of the IF preampliflier. If the 2A-B product level is more than 40 dB below the A or B level, then the system can be considered linear enough for use with the digital modulation techniques currently employed.
The addition of a GaAs FET preamplifier will reduce the system NF to 5 dB, but if no AGC is used ahead of the mixer the overload of the IF premplifier will become worse, actually decreasing the dynamic range of the receiver. Additional RF preamplification could be employed, but this has the effect of overloading the mixer which also adversely affects the available dynamic operating range. A variable attenuator could be inserted between the RF preamplifier and the mixer providing an AGC technique. The insertion of the loss in the RF portion of the receiver would necessitate the use of a second stage of preamplification in order to obtain the required low overall noise figure (NF). But now the second stage of the preamplifier will overload. One way to overcome this problem is to provide a variable gain RF preamplifier.